Re: [PATCH v4 0/6] UFFD write-tracking migration/snapshots

[Andrey Gruzdev <andrey.gruzdev@virtuozzo.com>]

On 26.11.2020 18:47, Peter Krempa wrote:
> On Thu, Nov 26, 2020 at 18:17:28 +0300, Andrey Gruzdev via wrote:
>> This patch series is a kind of 'rethinking' of Denis Plotnikov's ideas he's
>> implemented in his series '[PATCH v0 0/4] migration: add background snapshot'.
>>
>> Currently the only way to make (external) live VM snapshot is using existing
>> dirty page logging migration mechanism. The main problem is that it tends to
>> produce a lot of page duplicates while running VM goes on updating already
>> saved pages. That leads to the fact that vmstate image size is commonly several
>> times bigger then non-zero part of virtual machine's RSS. Time required to
>> converge RAM migration and the size of snapshot image severely depend on the
>> guest memory write rate, sometimes resulting in unacceptably long snapshot
>> creation time and huge image size.
>>
>> This series propose a way to solve the aforementioned problems. This is done
>> by using different RAM migration mechanism based on UFFD write protection
>> management introduced in v5.7 kernel. The migration strategy is to 'freeze'
>> guest RAM content using write-protection and iteratively release protection
>> for memory ranges that have already been saved to the migration stream.
>> At the same time we read in pending UFFD write fault events and save those
>> pages out-of-order with higher priority.
> 
> This sounds amazing! Based on your description I assume that the final
> memory image contains state image from the beginning of the migration.
> 
> This would make it desirable for the 'migrate' qmp command to be used as
> part of a 'transaction' qmp command so that we can do an instant disk
> and memory snapshot without any extraneous pausing of the VM.
> 
> I'll have a look at using this mechanism in libvirt natively.
> 

Correct, the final image contains state at the beginning of migration.

So far, if I'm not missing something about libvirt, for external 
snapshot creation it performs a sequence like that: 
migrate(fd)->transaction(blockdev-snapshot-all)->cont.

So, in case 'background-snapshot' capability is enabled, the sequence 
would change to:
stop->transaction(blockdev-snapshot-all)->migrate(fd).
With background snapshot migration it will finish with VM running so 
there's not need to 'cont' here.

Thanks, I'd appreciate a lot if you look how to integrate the new 
mechanism to libvirt!

-- 
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                 virtuzzo.com